We need inflation to make the Big Bang work, and that produces other universes.

For most of its history, the idea of a multiverse was the domain of science fiction and some rare speculation from physicists. In recent years, though, the idea that our Universe may be just one among many has gained traction in two different areas. The string theorists think it may help explain why, if there are a huge number of possible universes, we ended up in one with the properties we see around us. Meanwhile, cosmologists are realizing that inflation, which is the only way we know of to get from the Big Bang to our Universe, necessarily implies the creation of other universes.

Both of these threads of thought made appearances at two events held by the World Science Festival earlier this month, and different approaches to them were presented by various theorists—including two who think that we shouldn't be bothering with the multiverse at all.

The (contested) triumph of inflation

We'll start with inflation, which is the leading theory to explain why our Universe looks the way it does. The Universe is, on a large scale, homogeneous (matter is roughly equally distributed throughout). This implies that the Universe started small—small enough that different regions of it could equilibrate. But the Universe isn't perfectly smooth, as galaxies form clusters and other groupings. So how do you get from the small, equilibrated Universe to the existence of structure?

Inflation solves that problem, and two of the people who helped develop the theory (Alan Guth and Andrei Linde) were on hand to explain it. Inflation started when the Universe was small and even, with only random quantum fluctuations occurring within it. Then it rapidly blew it up over a tiny fraction of a second, etching the fluctuations into the form of the galaxy clusters and dark matter filaments that dominate the present Universe.

Inflation was one of a number of models that could potentially produce a Universe structured like our own, but it's been the last one standing since the results of the Cosmic Background Explorer came in; the recent Planck data only served to strengthen inflation's standing as the best way to explain how the Universe got its form. Linde said that when he first developed the idea of inflation, he never expected to see that sort of data. "I knew that it must be right, and I knew it wouldn't be tested," he told the audience. "I learned to respect our experimental colleagues." Meanwhile, Andreas Albrecht explained what the data meant for competing theories. Referring to fellow panelist Neil Turok, he said, "Neil and I worked hard on cosmic defect theories, and those were completely ruled out."

But if data on the cosmic microwave background killed his previous ideas, Turok wasn't ready to join the others in singing the praises of inflation. In fact, at one point he called it a "failed theory." Part of his issue is that inflation only gets you within a tiny fraction of a second of the Big Bang; it doesn't tell you what banged or how it got there. Guth didn't find that a persuasive argument at all, noting that most theories of the evolution of the Universe (including evolution itself) only trace the Universe's development from one point to another. Just as evolution doesn't need to explain the origin of life to be a success, inflation doesn't need to explain the Big Bang.

Infinite inflation

That technicality wasn't Turok's only issue. In general, he finds the implications of inflation unnecessarily complicated for what he sees as a simple Universe.

The problem (in Turok's view, at least) is that inflation necessarily produces a multiverse. In Linde's view, our Universe is just one spot on a three-dimensional fabric that's undergoing varying rates of inflation, perpetually spawning universes (we covered that in more detail in the past). However, you don't even need that fabric in order to create a lot of universes, as Guth's work has pointed toward "infinite inflation." In short, even if part of our Universe slowed its expansion, it's unlikely that the entire output of the Big Bang slowed down uniformly. Since inflation happens so fast, any part that kept inflating would spin off so much new material that even as more condensed into a universe, inflation would keep going with the remainders.

The end result is infinite inflation and an infinite number of what have been called "pocket universes." The general consensus on the panel was that if you liked inflation, you had to deal with a multiverse. As Guth put it in referring to the formation of multiple universes, "This is normal—no other mechanism in science ever works just once."

Of course, Turok doesn't like inflation, so that's fine with him. But Albrecht tried to occupy some ground in between the two. As inflation continues and accelerates due to dark energy, the Universe will eventually generate a sphere where the edge is a bit like the event horizon of a black hole: the fabric of space-time will be expanding fast enough that there is no possible way to ever reach anything beyond that. This, in Albrecht's view, might keep us from ever having to deal with any consequences of infinite inflation; in essence, inflation itself walls us off from any oddities caused by inflation.

If that was a possibility that interested Albrecht, the rest of the panels weren't all that interested in joining him. Some of that was because they're actually interested in the consequences of infinite inflation, but at least in Raphael Bousso's case, it's because a multiverse might help bail string theorists out of a rather thorny problem.

A multiverse on strings

String theory is currently an attempt to unify relativity and quantum mechanics within a single theoretical framework, but there was a time when people thought it could help get rid of another vexing question: why do some of the fundamental constants of the Universe have the values they do? If these had the wrong values, the Universe might have immediately reversed inflation and crunched back into a singularity or spread out so rapidly that no stars or galaxies could ever form.

It gradually became apparent that string theory was compatible with a lot of these values. In fact, current estimates are that string theory is compatible with any of 10500 different sets of values, each of which creates a distinct universe. To some extent, the multiverse gets string theory out of this bind. If enough universes are formed, we just happen to be living in the one that has the values we see through a mixture of random chance and its general compatibility with intelligent life.

Right now, though, we don't even know if this is an accurate view. As Bousso noted, we haven't been able to determine if any of the 10500 universes are more probable than others or if there are groups of values that create universes with similar properties. As Bousso put it, "we need to turn to statistical analyses, which is normal in physics."

Statistics has its own issues with a multiverse, though, creating what's called the measurement problem. Quantum mechanics indicates that the probabilities of some events have set values, but if there is an infinite number of universes with all these events happening over and over, then all the probabilities are essentially infinite. Which creates problems when we try to understand the measurements we see in our Universe. Again, Albrecht and Turok suggested that if you keep getting infinities, you must be doing something wrong. Bousso again pointed out that if you like inflation, you have to deal with the fact that inflation produces infinities.

Mario Livio, a physicist who moderated one of the sessions, summed things up with a comparison to Kepler's attempts to get the orbital distances of the planets to align with a group of Platonic shapes. "We consider that absurd, because we now know planetary orbits aren't fundamental properties of the Universe. But he was just doing what we're all doing today: trying to explain the Universe with the best math we have."